Fixes #10870

This issue occurs when the defaultOwner contains capital letters.
For example, if defaultOwner = Test, the userSpecificCommentField.name will be comment-Test, after saving it to the .bib file, it will be written as comment-test. As a result, every time the commentsTab is opened, it will contain both comment-Test and comment-test fields.

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.